Pipelined SAR with TDC converter

ABSTRACT

An analog-to-digital converter (ADC) is disclosed. The ADC has a successive approximation register (SAR) configured to receive an input signal and to generate a first digital signal having a plurality of most significant bits and a residue voltage. A voltage-to-time conversion element is configured to generate a time domain representation using the residue voltage. The voltage-to-time conversion element has an amplifier with a input coupled to an output of the SAR, a zero crossing detector coupled to an output of the amplifier, and a current source selectively coupled to the output of the SAR and the input of the amplifier by way of a switching element. A time-based signal processing element is configured to convert the time domain representation to a second digital signal having a plurality of least significant bits.

REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No. 15/590,247 filed on May 9, 2017, which is a Continuation of U.S. application Ser. No. 14/959,064 filed on Dec. 4, 2015 (now U.S. Pat. No. 9,673,835 issued on Jun. 6, 2017). The contents of the above-referenced matters are hereby incorporated by reference in their entirety.

BACKGROUND

An analog-to-digital converter (ADC) is a device that converts an analog input signal to a digital output signal comprising a sequence of bits that represent an amplitude of the analog input signal. An ADC typically has an analog reference voltage or current against which the analog input signal is compared. The digital output signal indicates what fraction of the reference voltage is the amplitude of the analog input signal. Analog-to-digital converters (ADCs) are widely used by modern day digital electronic systems to convert analog signals to digital signals that can be used by the digital electronic systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 illustrates a block diagram of some embodiments of a hybrid successive-approximation-register (SAR) analog-to-digital converter (ADC).

FIG. 2 illustrates some additional embodiments of a hybrid SAR-ADC.

FIG. 3 shows some embodiments of timing diagrams illustrating operation of the hybrid SAR-ADC of FIG. 2.

FIG. 4 illustrates a more detailed embodiment of a voltage-based signal processing element within a hybrid SAR-ADC.

FIG. 5 illustrates a more detailed embodiment of a time-based signal processing element within a hybrid SAR-ADC.

FIGS. 6A-6B illustrate a more detailed embodiment of a voltage-to-time conversion element within a disclosed hybrid SAR-ADC.

FIG. 7 illustrates a flow diagram of some embodiments of a method of performing an analog-to-digital conversion using voltage-based signal processing and time-based signal processing.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Successive-approximation-register analog-to-digital converters (SAR-ADCs) are a type of analog-to-digital converter commonly used in integrated chips. An SAR-ADC converts an analog input signal into a digital output signal having a plurality of bits via a binary search algorithm that converges on the analog input signal. Typically, SAR-ADCs may operate by way of a voltage-based signal processing. In voltage-based signal processing, the analog input signal is converted to a voltage, which is compared to a reference voltage to determine a first bit. After determining a first bit, the SAR-ADC moves to a next bit and performs another comparison to an updated reference voltage generated based upon the first bit. The sequence of comparisons continues until an n-bit digital word is generated.

As the scaling of integrated chips continues, the power efficiency of traditional ADCs using voltage-based signal processing decreases. This is because such ADCs (e.g., Pipelined ADCs) use high gain amplifiers that consume large amounts of power and which are not able to be readily scaled to emerging technology nodes (e.g., to a technology node having a minimum feature size of 16 nm, 10 nm, etc.).

The present disclosure relates to a hybrid SAR-ADC that uses a combination of voltage-based signal processing and time-based signal processing to convert an analog input signal to a digital output signal, and an associated method. In some embodiments, the hybrid SAR-ADC comprises a voltage-based signal processing element configured to convert an analog input signal to a first digital signal having a plurality of most significant bits (MSBs) and to generate a residue voltage from an input voltage and the first digital signal. A voltage-to-time conversion element is configured to convert the residue voltage to a time domain representation. A time-based signal processing element is configured to convert the time domain representation to a second digital signal comprising a plurality of least significant bits (LSBs). By determining the plurality of MSBs using voltage-based signal processing and determining the plurality of LSBs using time-based signal processing, the hybrid SAR-ADC is able to achieve a low power and a compact area.

FIG. 1 illustrates a block diagram of some embodiments of a hybrid successive-approximation-register analog-to-digital converter (SAR-ADC) 100.

The hybrid SAR-ADC 100 comprises a voltage-based signal processing element (SPE) 102 and a time-based signal processing element (SPE) 104, which are collectively configured to convert an analog input signal S_(IN) to a digital output signal D_(out). The voltage-based signal processing element 102 is configured to generate a first digital signal d_(MSB) corresponding to a plurality of most significant bits (MSB), while the time-based signal processing element 104 is configured to generate a second digital signal d_(LSB) corresponding to a plurality of least significant bits (LSB).

In some embodiments, the voltage-based signal processing element 102 comprises a successive-approximation-register (SAR) block 108. The SAR block 108 is configured to implement a binary search algorithm to determine digital values of the first digital signal d_(MSB). The digital values correspond to the analog input signal S_(IN) for the plurality of most significant bits (MSB). The SAR block 108 is also configured to generate a residue voltage V_(RES). The residue voltage V_(RES) corresponds to a difference between an input voltage determined from the analog input signal and a voltage corresponding to the first digital signal d_(MSB).

As the digital values for the first digital signal d_(MSB) are determined, the voltage converges on the analog input signal S_(IN), and the residue voltage V_(RES) (i.e., the difference between the input voltage and the reference voltage) decreases. The VTC element 110 is configured to generate a time domain representation T_(DIS) of the residue voltage V_(RES) by performing a voltage-to-time conversion of the residue voltage V_(RES) output from the SAR block 108. The time domain representation T_(DIS) comprises a pulse having a width that correspond to a value of the residue voltage V_(RES). For example, in some embodiments, a larger residue voltage V_(RES) may cause the time domain representation T_(DIS) to have a pulse with a greater pulse width than a smaller residue voltage.

The time domain representation T_(DIS) is provided to the time-based signal processing element 104. In some embodiments, the time-based signal processing element 104 may comprise a time-to-digital converter 112 configured to convert the time domain representation T_(DIS) to a second digital signal d_(LSB). The second digital signal d_(LSB) has digital values for the plurality of least significant bits (LSB).

The first digital signal d_(MSB) and second digital signal d_(LSB) are provided to a digital output signal generation element 106. The digital output signal generation element 106 is configured to combine the first digital signal d_(MSB) and the second digital signal d_(LSB) to generate a digital output signal D_(out). By employing a combination of voltage-based signal processing and time-based signal processing, the hybrid SAR-ADC 100 is able to achieve a low power and a compact area. This is because the time-based signal processing is able to work at low voltages that allow for the overall power of the hybrid SAR-ADC 100 to be low. Furthermore, the hybrid SAR-ADC 100 offers a resolution that improves with deep submicron scaling (e.g., to a technology node having a minimum feature size of 16, 10 nm, etc.), so as to provide the hybrid SAR-ADC 100 with scalability benefits over traditional SAR-ADCs.

FIG. 2 illustrates some additional embodiments of a hybrid successive-approximation-register analog-to-digital converter (SAR-ADC) 200.

The hybrid SAR-ADC 200 comprises a SAR block 202 configured to generate a first digital signal d_(MSB) corresponding to a plurality of most significant bits (MSBs) of an analog input signal S_(IN) The SAR block 202 comprises a track-and-hold circuit 204 coupled to an input node configured to receive the analog input signal S_(IN). The track-and-hold circuit 204 is configured to sample (i.e., capture) a value of the analog input signal S_(IN) and to hold the sampled value at a constant level for a period of time. The track-and-hold circuit 204 is configured to output the sampled value as an input voltage V_(IN) to an arithmetic unit 206. The arithmetic unit 206 is configured to determine a residue voltage V_(RES), which is equal to a difference of the input voltage V_(IN) and a DAC voltage V_(DAC) output from a capacitive digital-to-analog converter (CDAC) 212.

The residue voltage V_(RES) is provided from the arithmetic unit 206 to a comparator 208. The comparator 208 is configured to make a comparison to determine if the input voltage V_(IN) is greater than or less than the DAC voltage V_(DAC). For example, the comparator 208 is configured to generate a comparator signal S_(comp) based upon a comparison of the residue voltage V_(RES) and a ground potential GND. The comparator signal S_(comp) is used to determine a value of a next bit of the first digital signal d_(MSB). For example, if the input voltage V_(IN) is greater than the DAC voltage V_(DAC), the comparator signal S_(comp) will cause a next bit of the first digital signal d_(MSB) to have a first value (e.g., a “1”), while if the input voltage V_(IN) is less than the DAC voltage V_(DAC) the comparator signal S_(comp) will cause the next bit of the first digital signal d_(MSB) to have a different second value (e.g., a “0”).

The comparator signal S_(comp) is provided to a SAR logic unit 210. The SAR logic unit 210 is configured to generate control bits based upon the output of the comparator 208.

The CDAC 212 is configured to generate a new DAC voltage V_(DAC) based upon the control bits, which is used to determine a next bit of the first digital signal d_(MSB). In some embodiments, the control bits from successive iterations of a binary search algorithm implemented by the SAR block 202 may be stored in a register 211 within the SAR logic unit 210 and be provided as the first digital signal d_(MSB).

For example, to implement the binary search algorithm, the first digital signal d_(MSB) may be initially set to a value of “100000” (i.e., to have a most significant bit set to “1”). This value may cause the DAC voltage V_(DAC) to be equal to V_(REF)/2, where V_(REF) is a reference voltage provided to the CDAC 212. The arithmetic unit 206 determines a residue voltage V_(RES) from a difference between the DAC voltage V_(DAC) and the input voltage V_(IN), and the comparator 208 performs a comparison to determine if the input voltage V_(IN) is greater than or less than the DAC voltage V_(DAC). If the input voltage V_(IN) is greater than the DAC voltage V_(DAC), the comparator 208 outputs a high logical state (i.e., a “1”) and the MSB of the first digital signal d_(MSB) remains at 1. Conversely, if the input voltage V_(IN) is less than the DAC voltage V_(DAC), the comparator 208 outputs a low logical state (i.e., a “0”) and the MSB of the first digital signal d_(MSB) is changed to the logical low state. The SAR logic unit 210 then moves to a next bit and performs another comparison. The sequence continues for a number of bits until the first digital signal d_(MSB) comprises an n-bit digital word.

The residue voltage V_(RES) is also provided to a voltage-to-time conversion (VTC) element 214. The VTC element 214 is configured to perform a voltage-to-time conversion of the residue voltage V_(RES) to a time domain representation T_(DIS). The VTC element 214 comprises a current source element I_(DIS) 215 which is configured to discharge the reside voltage V_(RES) from the SAR block 202. The time taken to discharge the residue voltage V_(RES) is directly proportional to the value of the residue voltage V_(RES). In some embodiments, a residue offset circuit 217 is configured to provide a offset voltage V_(OS) to the arithmetic unit 206. The offset voltage V_(OS) is configured to cause the residue voltage V_(RES) to discharge in one direction regardless of whether a sign of the residue voltage V_(RES), so that the output of the VTC element 214 is linear.

In some embodiments, the VTC element 214 comprises an amplifier 218 that is configured to amplify the residue voltage V_(RES). The amplified residue voltage V_(RES)′ is then provided to a zero crossing detector 220. The zero crossing detector 220 is configured to convert the amplified residue V_(RES)′ to a time domain representation T_(DIS). In some embodiments, the zero crossing detector 220 may comprise a comparator configured to generate the time domain representation T_(DIS) by comparing the amplified residue voltage V_(RES)′ to a ground potential GND, so that the zero crossing detector 220 produces an output state change whenever the amplified residue V_(RES)′ crosses the ground potential GND. In some embodiments, the amplifier 218 may comprise an open loop amplifier. The use of an open loop amplifier minimizes power consumption of the voltage-to-time conversion (VTC) element 214.

The time domain representation T_(DIS) is provided to a time-to-digital conversion element 221 configured to convert the time domain representation T_(DIS) to the second digital signal d_(LSB). In some embodiments, the time-to-digital conversion element 221 may comprise a pulse generator 222 configured to receive the time domain representation T_(DIS) and to generate pulsed stop and start signals that are provided to a time-to-digital converter (TDC) 224. The pulsed stop and start signals are offset in time by an amount that is proportional to the time domain representation T_(DIS). The TDC 224 is configured to convert the pulsed stop and start signals to a second digital signal d_(LSB) corresponding to a plurality of least significant bits (LSBs) of the analog input signal S_(IN).

The first digital signal d_(MSB) and the second digital signal d_(LSB) are provided to a digital error correction element 226 configured to generate a digital output signal D_(out) therefrom. In various embodiments, the first digital signal d_(MSB) and/or the second digital signal d_(LSB) may have any number of bits. In some embodiments, the first digital signal d_(MSB) and/or the second digital signal d_(LSB) may have redundant bits, which can be used by the digital error correction element 226 to determined errors in the hybrid SAR-ADC 200. For example, the first digital signal d_(MSB) may comprise an 8-bit signal and the second digital signal d_(LSB) may comprise a 6-bit signal, which are combined by the digital error correction element 226 to generate a 12-bit digital output signal D_(out). In some embodiments, the digital error correction element 226 may comprise one or more shift registers configured to time-align bits from the first digital signal d_(MSB) and the second digital signal d_(LSB) prior to providing the bits to the digital-error-correction logic.

FIG. 3 shows some embodiments of timing diagrams 300 illustrating operation of the hybrid SAR-ADC 200 of FIG. 2.

Graph 302 shows the DAC voltage V_(DAC) and the input voltage V_(IN) (shown along the y-axis) as a function of time (shown along the x-axis). Graph 304 shows the residue voltage V_(RES) as a function of time. The residue voltage V_(RES) is equal to a difference of the DAC voltage V_(DAC) and the input voltage V_(IN) (i.e., V_(RES)=V_(DAC)−V_(IN)). As shown in graph 304, as the SAR block 202 determines more bits of the first digital signal d_(MSB), the residue voltage V_(RES) approaches zero (i.e., the DAC voltage V_(DAC) converges upon the input voltage V_(IN)).

Graph 306 illustrates the first digital signal d_(MSB) (i.e., the control bits of the SAR logic element). Initially, the first digital signal d_(MSB) is set to “100000.” During a first time period from t₀ to t₁ a first comparison is performed. Since the residue voltage V_(RES) is greater than 0 (i.e., the input voltage V_(IN) is greater than the DAC voltage V_(DAC)), the MSB remains at 1, as shown in graph 306. During a second time period, from t₁ to t₂, a second comparison is performed. Since the residue voltage V_(RES) is less than 0 (i.e., the input voltage V_(IN) is less than the DAV voltage V_(DAC)), a second bit remains at 0, so that the DAC is set to “100000.” During a third time period, from t₂ to t₃, a third comparison is performed. Since the residue voltage V_(RES) is greater than 0, a third bit is changed to “1”, so that the control bits of the SAR logic element are set to “101000.” Successive comparisons result in a first digital signal d_(MSB) having a value of “101000.”

Graph 308 illustrates a time domain representation T_(DIS) of the residue voltage V_(RES). The time domain representation T_(DIS) has a pulse comprising a width w that is dependent upon the residue voltage V_(RES). A second digital signal d_(LSB) is generated from the time domain representation T_(DIS). Since the time domain representation T_(DIS) corresponds to a residue of the SAR block 202, the second digital signal d_(LSB) corresponds to a LSB of the analog input signal.

FIG. 4 illustrates a more detailed embodiment of a hybrid successive-approximation-register analog-to-digital converter (SAR-ADC) 400.

The hybrid SAR-ADC 400 comprises a SAR block 401 and a VTC element 214. The SAR block 401 is configured to generate a first digital signal d_(MSB) corresponding to a plurality of most significant bits (MSB) of an input voltage V_(IN). The VTC element 214 is configured to receive a residue voltage V_(RES) from the SAR block 401 and to generate a time domain representation T_(DIS) of the residue voltage V_(RES). In some embodiments, a residue offset circuit 217 configured to generate an offset voltage V_(OS) is selectively coupled to the VTC element 214 by way of a switching element 406.

The SAR block 401 comprises a capacitive digital-to-analog converter (CDAC) 402 connected to an output of SAR logic unit 210. The CDAC 402 comprises a plurality of capacitors 404 coupled between a common terminal 403 and free terminals. The plurality of capacitors 404 have binary weighted values. The common terminal 403 is selectively coupled to a ground potential GND by way of a first switching element S₁. The free terminals are selectively coupled to the input voltage V_(IN) and/or one or more reference voltages V_(REFM) and V_(REFP) by way of additional switching elements SB.

During an acquisition phase, the common terminal 403 is held at the ground potential GND and the free terminals are connected to an input voltage V_(IN). After the acquisition phase, the common terminal 403 is disconnected from the ground potential GND and the free terminals are disconnected from the input voltage V_(IN), causing the plurality of capacitors 404 to hold a charge that is proportional to the input voltage V_(IN). The free terminals of the plurality of capacitors 404 are then connected to a minimum reference voltage V_(REFM) (e.g., ground), driving the common terminal 403 to a voltage equal to −V_(IN). The MSB capacitor is then disconnected from the minimum reference voltage V_(REFM) and connected to a maximum reference voltage V_(REFP), driving the common terminal 403 in the positive direction by an amount equal to ½V_(REF) so that V_(DAC)=−V_(IN)+½V_(REF). The comparator 208 is configured to output a 1 if V_(RES)<0 or a 0 if V_(RES)>0. If the comparator 208 outputs a 1, then the MSB capacitor 404 a stays connected to a maximum reference voltage V_(REFP). Otherwise, the MSB capacitor 404 a is connected back to the minimum reference voltage V_(REFM) (e.g., ground). The next smaller capacitor 404 b is then connected to the minimum reference voltage V_(REFM) and the new DAC voltage V_(DAC) is compared with the minimum reference voltage V_(REFM). This process continues until the first digital signal d_(MSB) has been determined.

FIG. 5 illustrates a more detailed embodiment of a time-based signal processing element 502 within a hybrid SAR-ADC 500.

The time-based signal processing element 502 is configured to receive a time domain representation T_(DIS) of a residue voltage V_(RES) from a voltage-to-time conversion (VTC) element 214. In some embodiments, the time-based signal processing element 502 may comprise a time-to-digital converter (TDC). The time-based signal processing element 502 comprises a tapped delay line 506 comprising a plurality of delay elements 506 a-506 n arranged in series to a pulse generator 504. The plurality of delay elements 506 a-506 n are configured to introduce a delay into a start signal S_(d0) generated by the pulse generator 504 to generate a plurality of delayed signals S_(d1)-S_(dn).

The plurality of delay elements 506 a-506 n respectively have an output coupled to a D-input of a corresponding flip-flop 508 a-508 n. The flip-flops 508 a-508 n further comprise a clock input. In some embodiments, the clock input is coupled to the time domain representation T_(DIS), which acts as a stop signal. In other embodiments, the clock input may be coupled to a pulse generator (not shown) configured to generate a stop signal that is delayed with respect to the start signal S_(d0) by an amount that is proportional to the time domain representation T_(DIS) of the residue voltage V_(RES).

The state of the tapped delay line 506 is sampled at a time of arrival of the time domain representation T_(DIS), such that a rising/falling edge of the time domain representation T_(DIS) (i.e., the stop signal) latches the status of the tapped delay line 506. In some embodiments, the output of the plurality of flip flops 508 a-508 n may be provided to a thermometer code converter 510 configured to generate the second digital output signal d_(LSB).

Since the VTC element 214 has represented the residue voltage V_(RES) as a time domain representation T_(DIS) in which the zero crossing point is indicated by a rising/falling edge, the time-based signal processing element 502 determines a values of the second digital signal d_(LSB) from when the rising/falling edge occurs (i.e., that indicates a digital value of the residue voltage V_(RES) using time domain signal processing). In other words, a time interval between start signal S_(d0) and a rising/falling edge of the time domain representation T_(DIS) is proportional to the number of flip-flops that were sampled as transparent, so that the outputs of the flip-flops 508 a-508 n define a second digital signal d_(LSB) having bits with values that depend upon the residue voltage V_(RES) (e.g., a first time difference may provide for a second digital signal d_(LSB) having a first sequence of bits (e.g., “1000”) while a second time difference may provide for a second digital signal d_(LSB) having a second sequence of bits (e.g., “1110”)).

FIGS. 6A-6B illustrates a more detailed embodiment of a voltage-to-time conversion element 602 within a hybrid SAR-ADC 600.

The hybrid SAR-ADC 600 comprises a SAR block 401 comprising a SAR logic unit 210 configured to generate a DAC control signal Φ_(DAC) having a plurality of bits. The DAC control signal Φ_(DAC) is provided to the CDAC 402 to control biasing of a plurality of capacitors 404 within the CDAC 402. Based upon the DAC control signal Φ_(DAC), the CDAC 402 will output a DAC voltage V_(DAC). The DAC voltage V_(DAC) is provided to an arithmetic unit 206 that is configured to subtract the DAC voltage V_(DAC) from an input voltage V_(IN) to generate a residue voltage V_(RES). The residue voltage V_(RES) is provided to a voltage-to-time conversion (VTC) element 602.

The VTC element 602 comprises an amplifier 218 connected to the arithmetic unit 206 and a zero crossing detector 220 connected to the amplifier 218. The amplifier 218 is further connected to a discharge current source 606 configured to generate a discharge current I_(DIS) that discharges the plurality of capacitors 404 within the CDAC 402. The time it takes to discharge the plurality of capacitors 404 is proportional to the residue voltage V_(RES). Therefore, when the zero crossing point of the residue voltage V_(RES) is reached, the zero crossing detector 220 will generate a time domain representation T_(DIS) comprising a pulse having a width that is a proportional to the residue voltage V_(RES). The time domain representation T_(DIS) may be subsequently converted to a second digital signal comprising the plurality of least significant bits.

In some embodiments, a residue offset circuit 604 is connected to an amplifier 218. The residue offset circuit 604 is configured to cause the residue voltage V_(RES) to discharge in one direction (e.g., to discharge from a positive value to zero or to discharge from a negative value to zero), regardless of whether the residue voltage V_(RES) is positive or negative. By causing the residue voltage V_(RES) to discharge in one direction, the time domain representation T_(DIS) is linear regardless of the characteristics of the amplifier 218.

The residue offset circuit 604 comprises a COS capacitive element 608. The COS capacitive element 608 is selectively coupled to a ground potential GND and to an offset voltage source V_(OS) by way of switching elements S₃-S₅. The COS capacitive element 608 is also coupled to the discharge current source 606 by way of switching element S₆. When the discharge current source 606 generates the discharge current I_(DIS), the COS capacitive element 608 also discharges to provide a residue offset voltage V_(OS) to the amplifier 218.

FIG. 6B illustrates timing diagrams 610 illustrating operation of the hybrid SAR-ADC 600 of FIG. 6A.

Graph 612 illustrates an input signal Φ_(TH) corresponding to the input voltage V_(IN) output from the track-and-hold circuit 204. The input signal Φ_(TH) is held at a value of an analog input signal for a period of time. For example, at time t₁ the input signal Φ_(TH) is held at a first value V_(IN1) for a time period that extends from time t₁ to time t₂. At other times the input signal Φ_(TH) is held low. At a first time t1, when the input signal Φ_(TH) is high, switching elements S₁-S₄ are closed, thereby initializing (e.g., connecting to GND) the capacitors within the CDAC 402 and also resetting the capacitance value of the COS capacitive element 608. At a second time t₂, when the input signal Φ_(TH) is low, switching elements S₁-S₄ are open, thereby disconnecting the capacitors from the ground potential GND.

Graph 614 illustrates DAC control signal Φ_(DAC) provided to the CDAC 402 to control biasing of a plurality of capacitors 404 within the CDAC 402. The DAC control signal is configured to generate a plurality of pulses Φ_(DAC,1)-Φ_(DAC,4), which sequentially bias the plurality of capacitors 404 in the CDAC 402 by operating switching blocks SB₁-SB₄ to connect the plurality of capacitors 404 to an input voltage and/or one or more reference voltages as described above.

Graph 616 illustrates a control bit signal Φ_(BIT) that operates switching element S₇ to selectively couple the comparator 208 to a ground potential GND. The comparator 208 performs a comparison on a falling edge of the control bit signal Φ_(BIT).

Graph 618 illustrates a residue voltage discharge signal Φ_(DIS) that operates switching element S₆ to selectively couple the discharge current source 606 to CDAC 402 and to COS capacitive element 608. For example, at a time t3, when the residue voltage V_(RES) is generated from the first digital signal d_(MSB), the residue voltage discharge signal Φ_(DIS) is set to high to couple the discharge current source 606 to the CDAC 402 and to the COS capacitive element 608. The discharge current causes charges from the plurality of capacitors 404 within the CDAC 402 and the COS capacitive element 608 to start discharging in a linear fashion. Once the plurality of capacitors 404 within the CDAC 402 have discharged and the residue voltage V_(RES) is equal to the ground potential GND, the amplifier 218 outputs a signal that causes the zero crossing detector 220 to generate a pulse that is proportional to the residue voltage V_(RES).

Graph 620 illustrates an amplifier control signal Φ_(A). The amplifier control signal Φ_(A) operates switching element S₈ to selectively couple the amplifier 218 to the SAR block 401 and the residue offset circuit 604. For example, at time t₄, the amplifier control signal Φ_(A) is set to high to close switching element S₈, so that a sum of the residue voltage V_(RES) and the residue offset voltage V_(OS) is provided to the amplifier 218. The resulting output voltage Vo of the amplifier is equal to Vo=[(V_(IN)−V_(DAC))+V_(OS)−I_(DIS)*T_(DIS)/C_(T)]*A_(OL), so that the zero-crossing of the output voltage Vo is detected at T_(DIS)=(V_(IN)−V_(DAC)+V_(OS))*C_(T)/I_(DIS) (where C_(T) is equal to the sum of the capacitors 404). Therefore, time domain representation T_(DIS) at zero crossing is linear regardless of the characteristics of the amplifier 218, so that the amplifier 218 may comprise a low power open loop amplifier with a moderately linear low gain.

Graph 622 illustrates an amplifier reset signal Φ_(CM). The amplifier reset signal Φ_(CM) operates switching element S₉ to selectively couple the amplifier 218 to a ground potential GND. For example, at time t₅, the amplifier reset signal Φ_(CM) is set to high to close switch S₉ so that the value of the time domain representation T_(DIS) goes low.

FIG. 7 illustrates a flow diagram of some embodiments of a method 700 of performing an analog-to-digital conversion using voltage-based signal processing and time-based signal processing.

While the disclosed method 700 is illustrated and described herein as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases.

At operation 702, an analog input signal is sampled to determine an input voltage.

At operation 704, a residue voltage is determined based upon a difference of an input voltage and a DAC voltage determined from the first digital signal.

At operation 706, a first digital signal comprising a plurality of most significant bits (MSBs) is determined from the residue voltage. In some embodiments, acts 702-706 may be repeated multiple times (as shown by arrow 708) to determine a plurality of bits for the first digital signal.

At operation 710 a residue offset voltage is added to the residue voltage, in some embodiments.

At operation 712, the residue voltage is converted to a time domain representation of the residue voltage.

At operation 714, the time domain representation is converted to a second digital signal comprising a plurality of least significant bits (LSBs).

At operation 716, the first digital signal and the second digital signal are combined to generate a digital output signal.

Therefore, the present disclosure relates to an analog-to-digital converter (ADC) that uses a combination of voltage-based signal processing and time-based signal processing to convert an analog input signal to a digital output signal, and an associated method of formation.

In some embodiments, the present disclosure relates to an analog to digital converter (ADC). The ADC includes a voltage-based signal processing element configured to receive an input signal and to generate a first digital signal having a plurality of most significant bits and a residue voltage. A residue offset circuit is configured to provide a residue offset voltage to the residue voltage, and a voltage-to-time conversion element is configured to convert a sum of the residue voltage and the residue offset voltage to a time domain representation. A time-based signal processing element is configured to convert the time domain representation to a second digital signal having a plurality of least significant bits.

In other embodiments, the present disclosure relates to an analog to digital converter (ADC). The ADC includes a voltage-based signal processing element configured to receive an input signal and to generate a first digital signal having a first number of most significant bits and a residue voltage. A voltage-to-time conversion element is configured to convert the residue voltage to a time domain representation, and a time-based signal processing element is configured to convert the time domain representation to a second digital signal comprising a second number of least significant bits. A digital error correction element is configured to receive the first digital signal and the second digital signal and to generate a digital output signal therefrom. The digital output signal has a third number of bits that is less than a sum of the first number and the second number.

In yet other embodiments, a method of performing an analog-to-digital conversion. The method includes generating a first digital signal having a first number of most significant bits based upon an analog input signal, and determining a residue voltage based upon the first digital signal. The method further includes adding a residue offset voltage to the residue voltage, and converting a sum of the residue voltage and the residue offset voltage to a time domain representation. The method further includes converting the time domain representation to a second digital signal having a second number of least significant bits.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. An analog-to-digital converter (ADC), comprising: a successive approximation register configured to receive an input signal and to generate a first digital signal having a plurality of most significant bits and a residue voltage; a voltage-to-time conversion element configured to generate a time domain representation using the residue voltage, the voltage-to-time conversion element comprising: an amplifier comprising an input coupled to an output of the successive approximation register; a zero crossing detector coupled to an output of the amplifier; and a current source selectively coupled to the output of the successive approximation register and the input of the amplifier by way of a switching element; and a time-based signal processing element configured to convert the time domain representation to a second digital signal having a plurality of least significant bits.
 2. The ADC of claim 1, further comprising: a residue offset circuit coupled to the output of the successive approximation register and to the input of the amplifier, wherein the residue offset circuit is configured to output a residue offset voltage.
 3. The ADC of claim 2, wherein the successive approximation register comprises a capacitive digital-to-analog converter having a plurality of capacitors collectively having a first capacitance.
 4. The ADC of claim 3, wherein the current source is configured to generate a discharge current that discharges the residue voltage from the successive approximation register.
 5. The ADC of claim 4, wherein an output voltage of the amplifier is equal to a product of a gain of the amplifier and a variable equal to a sum of the residue voltage and the residue offset voltage minus a product of the discharge current and the time domain representation divided by the first capacitance.
 6. The ADC of claim 4, wherein the time domain representation is substantially equal to the first capacitance divided by the discharge current and multiplied by a sum of the residue voltage and the residue offset voltage.
 7. The ADC of claim 1, wherein the amplifier comprises an open loop amplifier.
 8. The ADC of claim 1, wherein the time-based signal processing element comprises a pulse generator configured to receive the time domain representation and to generate pulsed stop and start signals that are provided to a time-to-digital converter, wherein the pulsed stop and start signals are offset in time by an amount that is proportional to the time domain representation.
 9. The ADC of claim 1, further comprising: a digital error correction element configured to receive the first digital signal and the second digital signal, and to generate a digital output signal; wherein the first digital signal comprises a first number of most significant bits and the second digital signal comprises a second number of least significant bits that is less than the first number of most significant bits; and wherein the digital output signal has a third number of bits that is less than a sum of the first number of most significant bits and the second number of least significant bits.
 10. The ADC of claim 1, wherein the time domain representation that is output from the voltage-to-time conversion element has a pulse width that is proportional to a value of the residue voltage.
 11. The ADC of claim 1, wherein the voltage-to-time conversion element has an output coupled directly to an input of the time-based signal processing element.
 12. An analog-to-digital converter (ADC), comprising: a voltage-based signal processing element configured to receive an input signal and to generate a first digital signal and a residue voltage; a residue offset circuit configured to provide a residue offset voltage to the residue voltage; a voltage-to-time conversion element configured to use the residue voltage and the residue offset voltage to generate a time domain representation of the residue voltage; and a time-based signal processing element configured to convert the time domain representation to a second digital signal.
 13. The ADC of claim 12, wherein the residue offset voltage causes the time domain representation to be linear.
 14. The ADC of claim 12, wherein the voltage-based signal processing element comprises a successive approximation register having a capacitive digital-to-analog converter including a plurality of capacitors collectively having a first capacitance.
 15. The ADC of claim 14, further comprising: a current source configured to generate a discharge current, wherein the current source is coupled to an output of the voltage-based signal processing element and to an input of an amplifier within the voltage-to-time conversion element; and wherein an output voltage of the amplifier is equal to a product of a gain of the amplifier and a variable equal to a sum of the residue voltage and the residue offset voltage minus a product of the discharge current and the time domain representation divided by the first capacitance.
 16. The ADC of claim 12, further comprising: a current source configured to generate a discharge current, wherein the current source is coupled to an output of the voltage-based signal processing element and to an input of the voltage-to-time conversion element; and a control unit configured to generate a first control signal that operates a first switching element to couple the current source to the output of the voltage-based signal processing element at a first time and further configured to generate a second control signal that operates a second switching element to couple the output of the voltage-based signal processing element to the input of the voltage-to-time conversion element at a second time after the first time.
 17. The ADC of claim 16, wherein the control unit is further configured to operate the first switching element to decouple the current source from the output of the voltage-based signal processing element at a third time and to operate the second switching element to decouple the voltage-to-time conversion element from the output of the voltage-based signal processing element at the third time.
 18. A method of performing an analog to digital conversion, comprising: generating a first digital signal from an analog input signal; determining a residue voltage from the analog input signal; generating a time domain representation by discharging the residue voltage over a time that is proportional to the residue voltage, wherein the residue voltage discharges in a same direction regardless of whether the residue voltage is positive or negative; and converting the time domain representation to a second digital signal.
 19. The method of claim 18, further comprising: coupling a current source to an output of a voltage-based signal processing element at a first time, wherein the voltage-based signal processing element is configured to receive the analog input signal and to generate the first digital signal; and coupling an input of a voltage-to-time conversion element to the output of the voltage-based signal processing element at a second time after the first time, wherein the voltage-to-time conversion element is configured to generate the time domain representation.
 20. The method of claim 19, further comprising: decoupling the current source from the output of the voltage-based signal processing element at a third time; and decoupling the voltage-to-time conversion element from the output of the voltage-based signal processing element at the third time. 